Poor Compression (80psi) = Engine Rebuild = FUN! (Right?)

[aedi006]

82' GS850GL
I've been putting off a compression test for a couple weeks now. I only bought the bike early in march and really didn't want to know if it had a bad ticker because I have it running now. However a sale at Harbor Freight ($5 Compression Tester!!!!!) inspired me. The results 1-4 in order 120, 80, 135, 140 psi (dry results). I then did a wet test on 2 (add oil to piston) and got 150psi so it's not the valves. So here comes my stream of questions......

How bad is removing the head with the engine still on the bike (hey, clymer says it can be done, how bad can it be)?

Whoever has done it before, can you bless me with some pointers?

What is the usualy suspect for bad compression in these engines, piston rings (please!), or worn out boar (I hope not, I don't want to have to machine this thing.... yet), or some choice "C" I haven't read about (remeber the wet test was good)?

What else should I do when taking the head apart? I already plan on glass bead blasting pretty much everything to clean it. Should I replace the valves while everything is apart? As soon as I finish this post I'm heading to bikebandit to see what kind of damage I can do.

Just in general.... any hints/tips/previous experiances would help me a lot. I have never taken apart a working engine before (I have trashed a couple of lawn mowers and chainsaws, but I was just bored). I bought this bike for just for times like this, I knew I could get it running, but I didn't want it to be easy, I love a good project. This is going to be a great week or two!!

[chef1366]

Has it sat for awhile? If you have a bad petcock it has leaked into the number 2 cylinder. I would change the petcock and run it for awhile and see if the rings seat.
Did you do the compression test with a warm engine and the throttle wide open?

[Nessism]

Engine can stay in the frame if you like.

The low compression is due to a poor ring seal. You need to pull the pistons and measure the cylinders before any judgement can be passed on the need to bore the cylinders. Just a guess, but if the engine has less than 50k miles you may well get away with just rings. Cylinder head is a similar story, pull everything apart and measure. Excess valve to guide clearance is not good and will require new guides and a valve job. To measure is to know (my signature )

[aedi006]

I have been driving the bike around for 2 weeks (apartently it can still go like stink on 3 cylinders). I had noticed that #2 didn't heat the exhaust at idle which helped push me towards the compression test. I did the test after a short ride and with the choke open and throttle wide. The bike has 24K on it.

[SkeletonLake]

I've had the engine on my '79 850 apart, right down to the crankshaft on the bench. I planned ahead for a change (novel concept, believe me). Went through the Cylmer page by page, writing down which the sections and paragraphs in sequence to teardown and reassemble. Didn't need special tools beyond a good torque wrench. Took my time, and it went back together just fine; even ran too! I agree you may not need to go this far, re-ringing #2 may do the trick. If you do tear down, buy a complete gasket kit and replace the output seal from the transmission to the drive gears.

[49er]

82' GS850GL
I've been putting off a compression test for a couple weeks now. I only bought the bike early in march and really didn't want to know if it had a bad ticker because I have it running now. However a sale at Harbor Freight ($5 Compression Tester!!!!!) inspired me. The results 1-4 in order 120, 80, 135, 140 psi (dry results). I then did a wet test on 2 (add oil to piston) and got 150psi so it's not the valves. So here comes my stream of questions......

How bad is removing the head with the engine still on the bike (hey, clymer says it can be done, how bad can it be)?

Whoever has done it before, can you bless me with some pointers?

What is the usualy suspect for bad compression in these engines, piston rings (please!), or worn out boar (I hope not, I don't want to have to machine this thing.... yet), or some choice "C" I haven't read about (remeber the wet test was good)?

What else should I do when taking the head apart? I already plan on glass bead blasting pretty much everything to clean it. Should I replace the valves while everything is apart? As soon as I finish this post I'm heading to bikebandit to see what kind of damage I can do.

Just in general.... any hints/tips/previous experiances would help me a lot. I have never taken apart a working engine before (I have trashed a couple of lawn mowers and chainsaws, but I was just bored). I bought this bike for just for times like this, I knew I could get it running, but I didn't want it to be easy, I love a good project. This is going to be a great week or two!!

Yes, engine rebuilding can be fun. It may take longer than 1-2 weeks though.
You have made a good start to the project by finding a Clymer Manual. The next thing to do is make sure you have good workshop space and the right tools to complete the job.
How many miles does your 850 have on it? Has it been used regularly or stored for a long period? If it was stored, your #2 cylinder's lower compression reading could have been caused by a damaged compression ring.
When the compression figures are even, you can usually get away with a re-ring and light honing of the bores. In your case, a rusted ring may have badly scored that #2 bore. If so, you may need to rebore.
Before you start, prepare a clean space for your cam cover,head and cylinders. Old newspapers make a good soft surface and are ideal for soaking up small amounts of oil.
It is easy to remove the cam cover, head and cylinders while the engine is still in the bike.
Follow the Clymer Manual instructions.
To gain adequate clearance, remove the breather/cam idler cover on top of the cam cover, before trying to remove the cam cover.

Bead blasting of the head needs to be done with caution. It is possible for beads to find their way into your engine if you dont get all the galleys,passages and stud holes blown clear with compressed air. Also, take care not to mark the valve bucket bores on the head. Many tuners, including myself won't take the risk with bead blasting.
I use a strong degreaser and then steam clean the head and cylinder assys.

While the head is off, remove the valves and inspect the seat areas on the valves and the pockets for burning and uneven wear. Also check the condition of the valve guides and valve stems for excessive wear.
Decarbon the chambers and ports and correct any casting/machining mis-matchs in the port and valve pocket/seat areas. Don't deliberately change the shape in these areas. If in doubt, ask.

[earlfor]

You might want to check valve clearances on the bad cylinder. Valves not quite closing is possible.

E.

I have been driving the bike around for 2 weeks (apartently it can still go like stink on 3 cylinders). I had noticed that #2 didn't heat the exhaust at idle which helped push me towards the compression test. I did the test after a short ride and with the choke open and throttle wide. The bike has 24K on it.

[Big T]

Have you checked the valve clearances?

Also, if it sat for a while, a dose of Marvel Mystery Oil in #2 couldn't hurt. The rings may just be stuck to the piston.

ALWAYS do the simple things 1st, and tear the engine apart 2nd

The engine can be torn down in the frame.

[Nessism]

Have you checked the valve clearances?

Also, if it sat for a while, a dose of Marvel Mystery Oil in #2 couldn't hurt. The rings may just be stuck to the piston.

ALWAYS do the simple things 1st, and tear the engine apart 2nd

The engine can be torn down in the frame.

I was thinking about suggesting the Marvel Oil trick as well. Helps free up stuck rings.

[Nightmare]

Eerie

Your problem is eeriely simillar to my bike...

http://www.thegsresources.com/_forum/showthread.php?t=103387&highlight=compression+question

I had low compression on the #3, but with only 31k on the bike i found it hard to believe the rings were worn... Not that this is a great deal of help, but I haven't been able to ride the bike long enough to be bothered retesting the compression to see if I have made any difference (stupid extended Canadian winter... just saw some more snow out the windows again...)

But if, I find a tick that clears up my low cylinder I'll post it for you!

[aedi006]

49er, thanks for the pointers, they will be taken to heart. I have had sucess bead blasting (using soft fine grain glass) the CAM cover when I re-shimmed the valves (so yes everybody, I have checked the valve spacing, 3 times). I was very careful to clean and air hose everything before re-assembly. The bike had sat before I aquired it, but I am not sure how long, somewhere between 2-7 years (bought it from a guy on ebay who bought it from a guy..... so no good history, but thats what you get with a $600 bike). So it is very possible that there could be rust. The petcock works fine, I haven't noticed any leaking when in the ON position. I did have flooding in #3 a week ago, but that was a stuck float (and my 3rd carb job). It is possible the flooding may have gotten gas into #2, but I changed the oil before running the engine and #3 seems fine.

My current plan of action:
I have MMO (thats some funky smelling stuff, brings back some childhood memeory I just can't place) sitting in #2 right now and will let it sit for 24 hrs. I will run the bike around this week (its my commuter) and next weekend will check compression again. If #2 is still crap the head will probally come off, but I will check with the forum first.

[ron bayless]

I can tell u and many others here will too that the gs petcocks are a big culprit for many problems like you described.After 20 plus years replace it and you solve many problems.Rebuild kits for petcocks are crap,dont buy one of those.Just my suggestions.